Is O2 sensor bank one- either the heater coil in the sensor is bad or you have a circuit problem in the vehicle wiring. You will need a wiring diagram to troubleshoot- on a 3 wire sensor, one is ground, one 12V and one signal I believe. You will also need a multimeter to properly diagnose.

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Dave

'05 325xi
'10 E70 35d
'85 911 Carrera
'66 911 Work in progress
'91 Chevy C1500
'95 Jeep Cherokee
‘18 Jeep JLU